Planning to cheat on your taxes? Embezzle corporate funds? Set off on a 10-state robbery spree? We won’t judge — but best get started after lunch. According to a new study published in Psychological Science, it’s easier to behave morally early in the day — and easier to misbehave the later it gets.

No matter how proud we may be of our own good character, moral behavior is not always a matter of having personal integrity and high ethical standards. Sometimes it’s just willpower. Loose cash lying about tempts us all. So does insider information we could use to make trades, or a cash register left unguarded while the 7-Eleven clerk runs to the back room. We resist plundering the till or making the illegal trade less because of the actual police officer on the beat (though that’s part of it) than the one in our brain — the suite of executive functions in our prefrontal cortex that regulate our baser impulses. In babies and children this takes instruction and practice. By the time we’re older, we check ourselves unconsciously.

But willpower is not infinite — something multiple studies of diets and substance-abuse recovery programs show. Like any other act of exertion, it can lead to fatigue, and when it does, we’re more inclined to slip. It’s the reason people in AA and Weight Watchers are taught not just to clamp the lid down on their appetites — a strategy that will inevitably fail — but to create entirely different lifestyles that don’t include alcohol or unhealthy foods.

To test how this exhaustibility of discipline affects moral choices, Maryam Kouchaki of Harvard University and Isaac Smith of the University of Utah, both Ph.D.’s in organizational behavior, recruited a sample group of volunteers and ran a number of morality experiments on them at two times of the day. One group was tested from 8 a.m. to noon and the other from noon to 6 p.m.

In the first experiment, the volunteers were shown a computer screen with dots arrayed on both the left and right halves, and were asked to determine which side had more. They were paid a small amount if there were more dots on the left side, and 10 times that amount if they counted more dots on the right. Significantly, they were not told that anyone would be checking their accuracy or their honesty. That, of course, made cheating look sweet, and afternoon fatigue indeed seemed to make it harder to resist that temptation. Those subjects who were tested in the noon-to–6 p.m. window cheated more than those in the 8 a.m.–to-noon slot.

“Mere time of day,” the researchers wrote, “can lead to a systemic failure of good people to act morally.” Not only that, time of day might also make people less inclined to think about — or even perceive — the idea of morality. In a second experiment, the subjects were given word fragments with blanks for missing letters, including “E _ _ _ C _ _” and “_ _ RAL” and asked to figure out what the word was. People tested earlier in the day were likelier to come up with “moral” and “ethical” than those tested in the afternoon, who were likely to guess words like “effects” and “coral.”

An additional portion of the study was conducted online, with volunteers being asked to solve a mathematical problem that in fact had no solution. They were told to report whether they’d gotten the answer or not, but they did not have to say what that answer was. Again, the later it was in the day, the greater the number of people who cheated and said they’d found a solution.

Kouchaki and Smith believe that the implications of what they call the morning-morality effect are obvious: when you find yourself faced with an opportunity to misbehave, be especially vigilant if it’s late in the day. And if you’re running a business or a classroom, keep a particularly close watch on your employees, customers and students during the afternoon. It’s never easy to be good — especially, it seems, as the hour grows late.